Sound dispersion device with internal divergent acoustical lens



Sept 10, 1957 F A MANLEY 2,805,728

SOUND DISPERSION DEVICE WITH INTERNAL DIVERGENT ACOUSTICAL LENS Filed Aug. 27, 1953 2 Sheets-Sheet 1 0 00000000000 O OOOOOQOOOOOO OO O O O O O '11 L 6 JNVENTOR.

FRED A. MANLEY X- HIS AGENT A. MANLEY SOUND DISPERSION DEVICE WITH INTERNAL DIVERGENT ACOUSTICAL LENS Filed Aug. 27, 1953 Sept 10, 1957 2 Sheets-Sheet 2 FIG INVENTOR. FRED A. MANLEY HIS AGENT ited States Patent Office SOUND DISPERSION DEVICE WITH INTERNAL DIV ERGENT ACGUSTICAL LENS Fred A. Manley, Rochester, N. Y., assignor, by mesne assignments, to General Dynamics Corporation, a corporation of Delaware Application August 27, 1953, Serial No. 376,815

10 Claims. (Cl. 18127) My invention relates to acoustic devices, and more particularly to acoustic devices for dispersing sound from relatively small sources, such as loudspeakers or similar transducers.

It is well known that high-frequency radiation from loudspeakers and similar transducers is concentrated in an axial beam. Divergent acoustic lenses mounted in front of transducers have been used to disperse these high frequencies over a wider area and thus obtain broader spatial coverage. These acoustic lenses have usually been mounted in front of a horn, such as an exponental horn. The high frequency transducers known as tweeters commonly have such horns.

Acoustic lenses have usually been housed in cylindrical containers attached to a flange adjacent the foremost flare of a horn. These cylindrical housings add undesired resonance effects and have a discontinuity at the joint between the cylindrical housing and the horn. The use of a separate cylindrical housing for the divergent acoustic lens makes a longer, heavier and clumsier unit, and the cost of the housing is an additional expense.

It is accordingly an object of my invention to provide a new and improved means for dispersing sound radiation.

It is also an object of my invention to provide anacous; tic device which disperses sound over a wide area, but which is compact and inexpensive.

It is a further object of my invention to provide an acoustic device which disperses sound over a wide area, yet which is free from resonances and the effects of discontinuities.

These and other objects are achieved according to my invention by a special acoustic lens placed within and adapted to cooperate With a horn to produce substantially the same acoustic result as the horn and cylindrical housing arrangement just described.

Further objects and advantages of my invention will become apparent as the following description proceeds and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of my invention, reference may be had to the accompanying drawing in which Fig. 1 is a front view of an acoustic lens in a cylindrical housing, as known before my invention;

Fig. 2 is a sectional view of the arrangement of Fig. 1;

Fig. 3 is a detail of Fig. 1;

Fig. 4 is a front view of one embodiment of my invention;

Fig. 5 is a sectional view taken along line 5-5 of the embodiment shown in Fig. 4; and

Fig. 6 is a cross-sectional view of a conventional loudspeaker embodying my invention.

My invention is best understood by referring first to the prior-art device shown in Figs. 1 and 2. This comprises an acoustic wave guiding means, such as a horn section 1, having a flange 2 at its throat, or inner end, and another flange 3 at its mouth, or outer, end. Horn 2,805,728 Patented Sept. 10, 1957 1 is defined by its enclosing walls which extend between flanges 2 and 3.

Flange 2 may be arranged to mate with a suitable transducer. The transducer is not shown because suitable transducers are well known to those skilled in the art and in any event, the nature of the transducer forms no part of my invention. Flange 3 mates with flange 4 of cylindrical housing 5. Flanges 3 and 4 may be secured together by any convenient means, such as bolts 6 and nuts 7.

An acoustic wave shaping means, such as acoustic lens 8, is mounted in cylindrical housing 5. Lens 8 is made up of a number of plane elements, of which elements 9 and 10 are identified by symbol for purposes of illustration. The elements are preferably uniformly spaced apart and may be secured in their relative positions by means of collars 11, screws 12 and nuts 13, as shown. The entire lens may be supported at the outer edge 14 of foremost lens element 10. This edge may be secured to the front surface of cylindrical housing 5 by any convenient means, such as by rolling the front edge 15 of cylinder 5 over edge 14. I prefer that the lens elements be made of a plastic material, such as cellulose acetate, to avoid the resonance effects which would be present if metal elements were used.

Each of the lens elements is perforated with a plurality of apertures, such as circular holes 16, for example, in element 10 (Fig. l). A preferred arrangement for the apertures is indicated in Fig. 3. The apertures may be arranged with their centers at the corners of an equilateral triangle, that is, a triangle having included angles of 60. The distance y between aperatures (see Fig. 3), is preferably less than a quater of a wavelength long at the highest frequency to be refracted through the acoustic lens. The inter-element spacing may also equal y (see Fig. 2). Where the highest frequency to be refracted is 13,500 cycles per second, y=% is satisfactory. The diameter 2 of the apertures may be /a for this frequency. An aid to lens design is contained in W. E. Keck and F. K. Harvey, Refracting Sound Waves, Journal of the Acoustical Society of America, vol. 21, No. 5, pp. 471-481.

A portion is cut out of the center of each lens element, the area removed being dependent upon the distance of the element from the front of the lens and increasing rearwardly. A curve drawn from cut edge to cut edge of these elements in a plane passing through the longitudinal axis (not shown) of lens 8 preferably has the hyperbolic shape indicated by curve 17.

The action of perforated plane lements in refracting sound waves is well known to those skilled in the art and need not be set forth here. Sufiice it to say that plane waves issuing from a transducer (not shown) adjacent flange 2 of horn 1 are delayed more at their outer edges than at their center, thereby changing the plane waves entering acoustical lens 8 into substantially spherical-fronted waves issuing from the front of lens 8.

It is immediately apparent that the overall length of horn 1 and cylindrical housing 5 cannot be shortened by discarding cylinder 5 and placing lens 8 within horn 1, because the diameter of lens 8 is too great. Moreover, the diameter of lens 8 cannot be cut down to fit into the horn, because too much of the lens body would be lost. In accordance with my invention, however, I provide a modified lens which fits into the mouth of the horn and cooperates with the horn to produce substantially the same acoustic result as the arrangement of Figs, 1 and 2.

Figs. 4 and 5 illustrate one embodiment of my invention. The enclosing walls of the acoustic wave guiding means, horn 18, may have the same shape as those of horn 1, and flange 19 may be arranged like flange 2 to mate with a suitable loudspeaker or other transducer.

No flange is necessary at the mouth end of horn 18 because the wave dispersing means, or lens, is within the horn, although a flange for mounting, not shown, may be provided if desired.

Acoustic lens 20, in accordance with my invention, comprises a plurality of plane elements, each of which is perforated with a plurality of apertures. The spacing of the plane elements, the sizes of the apertures, and the spacing of the apertures in each element may be similar to the corresponding factors employed with the arrangement of Figs. 1 and 2. In accordance with my invention, however, curve 21 which may be drawn in Fig. 5 from cut edge to cut edge of the adjacent elements, must have a different shape from that of curve 17.

I prefer that the shape of curve 21 be described graphically as follows: arrange the individual plane elements of lens 20 in the same spatial relationship as the elements of lens 8. Thus, element 22 of lens 20 is spaced the same distance as element 9 in lens 8 from the fronts of the respective lenses by the same distance, x. The maximum diameter of lens 8, represented by D, is made the same as the mouth diameter D of the horn. To determine the diameter of the cut out portion of an individual plane element, for example, d1 in element 22, the diameter of the corresponding element 9 in Fig. 2 is multiplied by the ratio of the inside diameter D1 of horn 18, at the place where element 22 is positioned, to D. In other words,

where d1 is the inside diameter of the cut out portion of a given element in Fig. 5, d is the diameter of the center cut out portion of the corresponding element in Fig. 2, D1 is the diameter of the horn at a point where the given element is situated and D is the mouth diameter of the horn. A similar determination is made for each of the elements in the lens.

It may be observed that curve 21 flares less' than curve 17. The optimum curve 17 for lens 8 is a hyperbola, but those skilled in the art can readily appreciate that other curves are employed in divergent acoustical lenses and that such curves may be the genesis for curve 21 using the method of curve description set forth above. I employ this method of description in this specification because a mathematical expression for curve 21 is awkward and difficult to derive. In essence, curve 21 is the product of curve 17, and the inner curve of horn 18.

My invention is not limited to use with horns, but may be employed with other acoustic wave guiding means such as the conical diaphragm 23 of loudspeaker 24, as sectionally shown in Fig. 6. In this case, lens 25 may be supported at the outer flange 26 of the basket 27 which supports conical diaphragm 23.

While I have shown and described my invention as applied to specific embodiments thereof, other modifications will readily occur to those skilled in the art. For example, some elements of the lens may have the same area of cut-out center portion, particularly where the curve from out edge to cut edge changes only slightly. I do not, therefore, desire my invention to be limited to the specific arrangements shown and described, and I intend in the appended claims to cover all modifications within the spirit and scope of my invention.

What I claim is:

1. In an acoustic device, the combination of acoustic wave guiding means having a mouth, a throat, and enclosing walls outwardly flaring from said throat to said mouth and a divergent acoustical lens within said guiding means adjacent said mouth.

2. The combination of claim 1 in which said lens is constructed of spaced-apart plane elements having formed therein a plurality of apertures each of said apertures having a maximum linear dimension smaller than a quarter of a wavelength of the highest frequency to be propagated through said lens.

3. The combination of claim 2 in which said elements are made of non-resonant material.

4. The combination of claim 2 in which each of at least some of said elements has a cut-out center portion larger in area than the total area of the apertures in that element.

5. The combination of claim 4 in which each of at least some of said elements has a said cut-out center portion of greater area than the cut-out portion of an element immediately adjacent thereto.

6. The combination of claim 5 in which a curve drawn from cut edge to cut edge of said elements in a plane passing through the longitudinal axis of said lens departs from hyperbolic shape in a predetermined relationship to the shape of the said wave guiding means.

7. The combination of claim 5 in which a curve drawn from cut edge to cut edge of said elements in a plane passing through the longitudinal aXis of said lens is the product of a hyperbolic curve and the curve of said wave guiding means.

8. The combination of claim 1 in which the flare varies according to a linear function.

9. The combination of claim 1 in which the flare varies according to a non-linear function.

10. The combination of claim 1 in which the flare varies according to an exponential function.

References Cited in the file of this patent UNITED STATES PATENTS 2,579,324 Kock Dec. 18, 1951 2,580,439 Kock Jan. 1, 1952 FOREIGN PATENTS 564,109 France Dec. 21, 1923 OTHER REFERENCES Journal of the Acoustical Society of America. Refracting Sound Waves by Kock, volume 21, No. 5, Sept. 1949, pages 471-481.

The New Jim Lansing Acoustical Lens, Mar. 18, 1952, published by James B. Lansing Sound Inc. 2 pages, 2439 Fletcher Drive, Los Angeles, Calif. 

